Motorola 6864115B62-C, Detailed Service Manual

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2-6 THEORY OF OPERATION
U4502 is a temperature-sensing device, which monitors the circuit board temperature in the vicinity
of the transmitter driver and final devices, and provides a dc voltage to the PCIC (TEMP, pin 30)
proportional to temperature. If the DC voltage produced exceeds the set threshold in the PCIC, the
transmitter output power will be reduced so as to reduce the transmitter temperature.
4.0 UHF (403-470MHz) Frequency Synthesis
The synthesizer subsystem consists of the reference oscillator (Y4261 or Y4262), the Low Voltage
Fractional-N synthesizer (LVFRAC-N, U4201), and the Voltage Controlled Oscillator VCO.
4.1 Reference Oscillator
The reference oscillator (Y4262) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An Analogue to Digital (A/D) converter internal to U4201 (LVFRAC-N) and
controlled by the microprocessor via serial interface (SRL) sets the voltage at the warp output of
U4201 pin 25 to set the frequency of the oscillator. The output of the oscillator (pin 3 of Y4262) is
applied to pin 23 (XTAL1) of U4201 via a RC series combination.
In applications where less frequency stability is required the oscillator inside U4201 is used along
with an external crystal Y4261, varactor diode D4261, C4261, C4262 and R4262. In this case,
Y4262, R4263, C4235 and C4251 are not used. When Y4262 is used, Y4261, D4261, C4261,
C4262 and R4262 are not used, and C4263 is increased to 0.1 uF.
4.2 Fractional-N Synthesizer
The LVFRAC-N synthesizer IC (U4201) consists of a pre-scaler, a programmable loop divider,
control divider logic, a phase detector, a charge pump, an A/D converter for low frequency digital
modulation, a balance attenuator to balance the high frequency analogue modulation and low
frequency digital modulation, a 13V positive voltage multiplier, a serial interface for control, and
finally a super filter for the regulated 5 volts.